1. Field of the Invention
The present invention concerns improvements to switching networks and, more particularly, to improvements in switching networks employing several stages of switching matrices such as those used in space switching telephone or telegraph exchanges.
2. Description of the Prior Art
A switching matrix has a first group of accesses which arbitrarily may be called inputs, a second group of accesses which may be called outputs and a plurality of crosspoint contacts between the accesses. The crosspoints make it possible to connect each input to all or some outputs and each output to all or some inputs.
A switching network comprising several selection stages has also inputs and outputs, and it is constituted by an arrangement of interconnected matrices. In a general way, the network inputs are connected to the inputs of a certain number of matrices constituting a first selection stage. The outputs of the first stage matrices are connected each by a "link" to the inputs of a certain number of matrices constituting a second selection stage. Similarly, the outputs of the second stage matrices are connected to the inputs of the third stage matrices, and so on. The network outputs are connected to the outputs of the last stage matrices.
Such an arrangement not only enables the connection from a network input to any network output, by a connection path including a matrix of each stage and the corresponding links between stages, but also offers between the considered input and output, the greatest possible number of separate connection paths, in order to have the greatest possibility of satisfying any connection request, and, finally, in that the network handles the traffic in the best conditions.
Such traffic considerations would lead to distribute outputs of each matrix of any stage on all the inputs of the next stage and, reciprocally, to distribute the inputs of each matrix of this next stage on all the outputs of the first considered stage, performing what will be called a general grading. However, experience and calculation have shown that such a general grading was not necessary for all the network stages. The switching networks are therefore generally constituted from selection units.
A selection unit includes a defined number of first stage matrices, a defined number of second stage matrices and links, interconnecting, inside the selection unit, the outputs of the first stage matrices to the inputs of the second stage matrices. In this case, a two-stage selection unit is concerned. In the same way, a selection unit with three stages or more may be designed.
A four-stage switching network may thus be composed of two-stage line selection units constituting the first two stages of the network, then of two-stage group selection units constituting the last two stages of the network. An example of such a switching network is described in the paper "Pentaconta Dial Telephone Switching System" of the "Electrical Communication" American revue of June, 1954.
An advantage of such a design is that switching networks of increasing dimensions can be built by merely adding standard selection units and that, particularly, it avoids any modification of the existing selection units when extending the network. However, it is necessary that there subsists a general grading between the outputs of the line selection units and the inputs of the group selection units. Indeed, it is preferable that the outputs of a line selection unit matrix be connected to the different group selection units and that the inputs of a group selection unit matrix be connected to the different line selection units. This general grading is defined for each network and must be modified when an extension is made.
A general grading of such type as described above is usually done by means of an intermediate distributing frame. The outputs of the previous stage are connected to tags "outputs" of the intermediate distributing frame. The inputs of the next stage are connected to tags "inputs." A removable wiring connects each "output" to each "input," in order to carry out the grading provided in each application. This wiring is modified and completed when an extension is made.
However, French patent application No. 70 11616, filed on Apr. 1, 1970 by the applicant, for "Improvements to switching networks" and published in France as Pat. No. 2,086,535, corresponding to U.S. Pat. No. 3,710,030, issued Jan. 9, 1973 provides the means of distributing the outputs of the line selection elements into output groups, of distributing the inputs of the group selection units into input groups, and carrying out a general grading between the line selection unit outputs and group selection unit inputs, whatever may be the network dimensions, by connecting the output groups to the input groups by means of plug-in cables associating each group of outputs to a group of inputs.